The signs of a blown head gasket aren’t always obvious. Knowing exactly how the head gasket works can help steer you in the right direction when it comes to making a successful diagnosis and repair.
In addition to electrical sensors and actuators, today’s internal combustion engine relies on liquids and gases to function: air-and-fuel, engine coolant and engine oil. Engine design keeps these liquids and gases from mixing so they can perform their specific functions. The head gasket is mounted between the engine block — where the cylinders are — and the cylinder head — where the intake, exhaust and valves are — and performs several critical functions.
What the Head Gasket Does
Upon combustion, air and fuel can generate upward of 700 psi in gasoline engines and upward of 2,000 psi in diesel engines. In order to keep that pressure in the combustion chamber, the head gasket needs to be robust and installed properly. The head gasket prevents liquids and gases from escaping into adjacent cylinders and the surrounding oil and coolant galleries.
Engine coolant surrounds each cylinder in order to maintain a stable operating temperature; it also needs to flow into the cylinder heads to cool the combustion chamber, valve and spark plugs. The head gasket prevents coolant from entering the cylinders between power strokes and when the engine is off, and the oil (when the engine is off).
Engine oil performs three important functions: lubrication, cooling and hydraulics. The oil pump sends pressurized oil — up to 60 psi in most applications — throughout the engine to lubricate bearings, bushings, journals and timing chains. It also drives hydraulic actuators, such as variable valve timing. The head gasket prevents oil from entering the cylinders, between power strokes, and the coolant.
When a Head Gasket Fails
The signs of a blown head gasket can be subtle. Here are eight of the most common indications that your head gasket has failed:
- An external oil or coolant leak at the seam between the engine block and cylinder head is a sign that you have a head gasket failure or a cracked block. On disassembly, check for cracks and cylinder head warping.
- Cylinder misfire is another sign of head gasket failure, especially if the breach is between two cylinders on the same head. Cylinder compression and leakdown tests can localize the leak for scrutiny on disassembly.
- Misfire on startup, if accompanied by a puff of white exhaust, could indicate a leak from the cooling system into the cylinder.
- Overheating is one of the less-obvious signs of a blown head gasket. Depending on the severity of the leak, you may see bubbles in the overflow tank, indicating that the cylinder is leaking into the cooling system.
- Blue exhaust smoke could also indicate head gasket failure, but so could worn rings, valve stem seals or a faulty PCV system.
- Discolored fluids are more subtle signs of a blown head gasket.
- Coolant-contaminated oil takes on a frothy consistency; it’s like finding a latte under your oil cap or in the valve covers.
- Oil-contaminated coolant forms a mayonnaise-like film, which you might find on the radiator cap or in the overflow reservoir.
If you suspect you have a blown head gasket, take your time diagnosing the problem. Dry and wet compression tests and cylinder leakdown tests might reveal the location of the leak. A block tester can help determine whether combustion gases are present in the cooling system — a sure sign of a head gasket failure. Your local NAPA AutoCare Center has the right equipment to help diagnose a blown head gasket if you have any suspicions.
Check out all the engine gaskets available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on signs of a blown head gasket, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
Photo courtesy of Flickr.
Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.